1. Field of the Invention
The present invention relates to a lens for projection and a projection-type display apparatus, and particularly to a lens for projection substantially consisting of six lenses and a projection-type display apparatus using the lens for projection.
2. Description of the Related Art
In recent years, projection-type display apparatuses (also called as projectors) rapidly spread. The projection-type display apparatuses project images represented by image data output from personal computers or the like onto screens. As such projectors, a projector that projects, onto a screen, light that has been output from a light source and modulated by a light valve is known. As the light valve, a transmission-type liquid crystal device, a reflection-type liquid crystal device, a DMD (Digital Micromirror Device), and the like are known. In the DMD, micromirrors, the angles of which are changeable, are arranged.
Such a projector needs to have a space for structuring an optical system for guiding light output from the light source to the light valve and an optical system for guiding light modulated by the light valve to the lens for projection. Therefore, a lens for projection to be mounted on the projector has a long back focus so that such a space is provided between the lens for projection and the light valve.
For example, a projector for projecting color images, and which includes light valves for modulating red light, green light and blue light respectively, needs to have a space between the lens for projection and the light valves to arrange optical systems for guiding light of each color to respective light valves and optical systems for guiding light modulated by the light valves to a lens for projection.
For example, as a lens for projection in which a back focus for securing such a space is set, a lens for projection composed of six lenses, and which has an F-number in the range of 1.8 to 2.8 is known (please refer to Japanese Unexamined Patent Publication No. 2001-124988 (Patent Document 1), Japanese Unexamined Patent Publication No. 2005-164839 (Patent Document 2), and Japanese Unexamined Patent Publication No. 2009-210596 (Patent Document 3)).
Further, as a lens for projection composed of six lenses, and which has a small F-number (fast lens), a lens for projection with an F-number of 1.7 (please refer to Japanese Unexamined Patent Publication No. 2006-309076 (Patent Document 4), and a lens for projection with an F-number of 1.5 (please refer to Japanese Unexamined Patent Publication No. 2004-184932 (Patent Document 5)) are known.
Here, when the number of lenses is counted, if a cemented lens composed of n lenses is included, the number of lenses of the cemented lens is regarded as n.
When images are projected onto a screen through a projector, a setting place of the screen is generally made dark, but a demand for watching images without making the setting place very dark is strong. However, projectors in which lenses for projection having relatively large F-numbers, as disclosed in Patent Documents 1 through 3, are mounted do not meet such a demand. Meanwhile, a projector in which a lens for projection having a small F-number (fast lens), as disclosed in Patent Documents 4 and 5, is mounted may meet the demand, but the reduction in the F-number (faster lens) causes a problem that the size of the apparatus becomes large, and a problem that it becomes difficult to suppress generation of chromatic aberrations.
More specifically, the lens for projection (F-number: 1.7) disclosed in Patent Document 4 has a problem in the power balance of lenses. Therefore, the size of a magnification-side lens or lenses is large, and the back focus is small. Further, in the lens for projection (F-number: 1.5) disclosed in Patent Document 5, correction of spherical aberrations, especially chromatic aberrations of marginal rays, is insufficient.